Dry developer for electrostatic copying

ABSTRACT

A DEVELOPER MIX FOR ELECTROSTATIC IMAGES CONSISTING ESSENTIALLY OF A TONER AND CARRIER OF IRON PARTICLES, SAID TONER CONSISTING ESSENTIALLY OF A POWDER REACTION PRODUCT OF (1) ETHYL CELLULOSE, POLYSTYRENE OR MIXTURES THEREOF (2) CARBON BLACK OR AZO OIL BLACK AND (3) SULFUR. ALSO PROVIDED IS AN ELECTROSTATIC PRINTING PROCESS FOR DEVELOPING A LATENT ELECTROSTATIC IMAGE TO YIELD AN ELECTROSTATICALLY-ADHERING IMAGE OF ELECTROSTATICALLY-ATTRACTABLE POWDER COMPRISING MAGNETICALLY TRANSPORTING ACROSS THE LATENT ELECTROSTATIC IMAGE A DRY ELECTROSCOPIC DEVELOPER MIXTURE COMPRISING A TONER CONSISTING OF LOOSE MOVABLE PARTICLES OF ELECTROSTATICALLY-ATTRACTABLE POWDER AND A CARRIER COMPRISING SEPARATE MAGNETICALLY ATTRACTABLE GRANULES, THE MAGNETICALLY-ATTRACTABLE GRANULES AND POWDER HAVING A TRIBOELECTRIC RELATIONSHIP OF OPPOSITE POLARITY, THE POWDER PARTICLES THEREBY BEING ELECTROSTATICALLY CHARGED THROUGH TRIBOELECTRIC ACTION BY CONTACT WITH THE MAGNETICALLY-ATTRACTABLE GRANULES TO ADHERE ELECTROSTATICALLY TO THE SURFACE OF THE MAGNETICALLY-ATTRACTABLE GRANULES AND BEING ATTRACTABLE BY CHARGED AREAS OF THE LATENT ELECTROSTATIC IMAGE WHEN MOVED THEREACROSS, AND THE MAGNETICALLY ATTRACTABLE GRANULES BEING CORRESPONDINGLY ELECTROSTATICALLY CHARGED TO OPPOSITE POLARITY AND THUS ADAPTED TO ATTRACT THE CHARGED PARTICLES AND REMOVE THEM FROM UNCHARGED AREAS OF THE LATENT ELECTROSTATIC IMAGE WHEN MOVED THEREACROSS, THE ELECTROSTATICALLY-ATTRACTABLE POWDER COMPRISING A MIXTURE OF THERMOPLASTIC RESINS, PIGMENTS AND DYES HAVING ABOUT 1 TO 60 WEIGHT PERCENT OF SULFUR INCORPORATED THEREIN.

United States Patent 01 US. Cl. 25262.1 Claims ABSTRACT OF THE DISCLOSURE A developer mix for electrostatic images consisting essentially of a toner and carrier of iron particles, said toner consisting essentially of a powder reaction product of (l) ethyl cellulose, polystyrene or mixtures thereof (2) carbon black or azo oil black and (3) sulfur.

Also provided is an eletrostatic printing process for. developing a latent electrostatic image to yield an electrostatically-adhering image of electrostatically-attractable powder comprising magnetically transporting across the latent electrostatic image a dry electroscopic developer mixture comprising a toner consisting of loose movable particles of electrostatically-attractable powder and a carrier comprising separate magnetically attractable granules, the magnetically-attractable granules and powder having a triboelectric relationship of opposite polarity, the powder particles thereby being electrostatically charged through triboelectric action by contact with the magnetically-attractable granules to adhere electrostatically to the surface of the magnetically-attractable granules and being attractable by charged areas of the latent electrostatic image when moved thereacross, and the magnetically attractable granules being correspondingly electrostatically charged to opposite polarity and thus adapted to attract the charged particles and remove them from uncharged areas of the latent electrostatic image when moved thereacross, the electrostatically-attractable powder comprising a mixture of thermoplastic resins, pigments and dyes having about 1 to 60 weight percent of sulfur incorporated therein.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to electrostatic printing procedures and more particularly to an improved toner mixture for use in developer mixes in electrostatic printing.

Description of the prior art In procedures for elfecting electrostatic printing, a latent image in the form of an electrostatic charge is usually produced upon a surface comprising a photoconductive insulating substance, such as for example selenium. An explanation of this process and its principles may be found in an article entitled Xerography, a New Principle of Photography and Graphic Reproduction by Shaifert and Oughten in the Journal of the Optical Society of America, December 1948, at page 991. According to this procedure, the latent image so formed "Ice is developed by allowing a powder comprising colored particles, often called a toner, having a suitable electrostatic charge, to come into contact with the image-bearing surface. The particles of powder may be charged by any ordinary means such as by contact with another charged surface or they may be charged by means of the triboelectric efiect by contact with carrier particles of another material separated from the powder in the triboelectric series, the latter sometimes referred to as a carrier. These colored particles are retained on portions of the surface by electrostatic forces in the charged area. The uncharged areas do not retain the particles but remain clear, thus forming the image. This invention is concerned with effecting the charged in the latter manner, i.e., by use of the triboelectric eifect.

In the practical aspects of carrying out electrostatic printing processes, a magnetically-attractable carrier material comprising large particles of some material separated in the triboelectric series from the electrostaticallyattractable toner is usually employed with the toner in forming the developer powder. In this aspect, the carrier and the powder are held together by the electrostatic forces produced upon contact between them by the triboelectric elfect. The usual method for developing the charged surface having a latent image with this developer mix is to fiow the mixed carrier and powder, as by a brush, across the surface so that the powder is removed from the carrier by, and retained on, the charged areas of the surface. The developer mix comprising the carrier and the powder must be brought into contact with the surface containing the latent image under light excluding conditions.

As pointed out, the dry developer mix usually comprises a toner in combination with a carrier material. The toner is usually a mixture of various thermoplastic resins with pigments or dyes and a common carrier which has been used heretofore is iron powder. The triboelectric properties of the toner, which reside primarily in the resins used, is such that, as the particles of toner and particles of iron rub against each other in normal handling, an electric charge is built up on the particles of the toner. If the toner is above iron in the triboelectric series, a positive charge is built up on the particles of the toner. Conversely, if the toner is below iron in the triboelectric series, a negative charge will be built up on the particles of toner. In simple terms, the expression triboelectric of toner. In simple terms, the expression triboelectric may be considered merely the ability of two dissimilar materials when rubbed together to generate charges of static electricity on the materials which are being rubbed together. The farther apart the two materials are in the triboelectric series, normally the stronger the charge which is built up. It is therefore necessary, in order to obtain good results in electrostatic printing, to avoid the use of toners which contain thermoplastic materials which are too close in the triboelectric series to the carrier material, which, as pointed out above, are ordinarily particles of iron. Quite often, certain products which are below the carrier in the triboelectric series will be used and therefore a negative will be acquired in rubbing so that when used, the toners are given a negative charge and will produce a reverse image, e.g., one in which the letters are white and the background black.

In the electrostatic printing process however, numerous problems have been encountered in devising toner materials which will exhibit satisfactory printing properties when applied from suitable iron carriers. Heretofore, it has been found that to have satisfactory toner materials, they should exhibit suitable triboelectric properties, have good thermostability, possess relatively sharp melting points at temperatures which are compatible with the backing material and have good adhesion to the backing material. To the present time however, no toner material is available which exhibits all of the above properties and yet exhibits good printing properties while being inexpensive. Therefore, it is apparent that a distinct need remains in the art for suitable toner materials which satisfy the above requirements for use in electrostatic printing.

SUMMARY OF THE INVENTION It is accordingly one object of the present invention to provide new dry developer compositions and processes for their use in electrostatic copying which overcomes or otherwise mitigates the problems inherent in similar compositions and processes known to the prior art.

Another object of the invention is to provide new dry developer compositions for developing latent electrostatic images wherein the developer compositions include improved toner materials which may be inexpensively produced.

A further object of the present invention is to provide an improved sulfur-containing toner mixture for use in dry electroscopic developer mixes in electrostatic printing.

A still further object of the present invention is to provide an improved toner mixture for combination with a carrier in producing a developer mix for electrostatic printing wherein the toner exhibits good triboelectric properties, good thermostability, a relatively sharp melting point at temperatures compatible with the backing material, and good adhesion of the toner to the backing material which contains about 1 to about 60 weight percent of sulfur.

Other objects and advantages of the present invention will become apparent as the description thereof proceeds.

In satisfaction of the foregoing objects and advantages, there is provided by this invention an improved dry developer mix for use in developing latent electrostatic images, said mix comprising a toner and a carrier therefor, said toner comprising a dry mixture of loose movable particles comprising thermoplastic resins, coloring agents and about 1 to 60 weight percent of sulfur.

Also provided by this invention is an electrostatic printing process for developing a latent electrostatic image to yield an electrostatic adhering image of electrostaticallyattractable powder comprising, employing a developer mix therein which comprises a toner and a carrier, said toner comprising colored thermoplastic particles having about 1 to about 60 weight percent of sulfur incorporated therein.

DESCRIPTION OF PREFERRED EMBODIMENTS In accordance with the preferred aspects of the present invention, it has been found that in procedures for electrostatic printing, the incorporation of sulfur into a certain mixture of resins, pigments and dyes affords excellent toner materials and gives satisfactory printing effects. Thus this invention provides a new dry developer mix having an improved toner mixture and a novel electrostatic printing process utilizing the novel developer mix.

In a typical electrostatic printing operation, a paper surface, which has adhered thereto a coating of zinc oxide in a thermoplastic resin, has imparted thereto a negative charge over the entire surface of the sheet by subjecting it to corona discharge. The charged sheet is then exposed to light under a transparency or by reflection through a lens and mirror system. In those portions of the negatively charged sheet which are hit by light, the negative 4 charge is neutralized or dissipated so that only those portions which are not hit by light (i.e., those which contain printing), still retain a negative charge. The developer mixture of carrier and toner is then dusted on the exposed surface of the sheet with the result that the positively charged particles of toner adhere to the negatively charged portions of the sheet but do not adhere to the portions of the sheet where the charge has been dissipated (i.e., the background portions of the sheet).

It is customary to apply the developing powder, which is usually a mixture of a carrier, such as iron powder, which is magnetically-amactable, and toner particles, which are electrostatically-attractable, by contacting the surface of the exposed sheet with a magnetic brush which has been dipped in or dusted with the developer powder. It is also customary to simultaneously apply to the back of the sheet a positive charge, which, while it somewhat weakens the negative charge remaining on the printed portions of the sheet, does not completely neutralize them but does serve to repel the positively charged toner from the uncharged background portion of the sheet.

After the developer mix has been applied to the exposed sheet, and is adhering to the negatively charged portions thereof, the entire sheet is heated sufiiciently so as to soften the coating on the sheet as well as the plastic in v the toner and to permanently fuse or :62: the toner on the sheet so that the letters will not be removed by merely rubbing the paper.

Toners which are employed in this process generally include a mixture of thermoplastic resins in combinations with various pigments and dyes as coloring agents. According to the present invention, it has been found that ethyl cellulose and low molecular weight polystyrene, and mixture of the same, are particularly suitable for this purpose and thus these materials represent a preferred embodiment of the invention. A particularly suitable class of polystyrenes for use in the present invention include those sold under the trade name Piccolastic, by the Pennsylvania Industrial Chemical Corporation, of Clairton, Pa. These Piccolastic resins are a series of resins based on pure styrene raw materials whose production is controlled to yield resins of intermediate molecular weight as opposed to the extremely long chains which are found in molding grades of polystyrene.

The polystyrenes which are suitable as resins in the present invention are those which have melting points in the range of about 50 C. to about 150 C. and have molecular weights ranging from about 350 to about 5000. Especially preferred polystyrenes for use in the present invention are those having melting points ranging from about 50 to about C. with molecular weights of about 350 to 1500. An especially preferred polystyrene is Piccolastic D75 which melts at about 75 C. with a molecular weight of about 800.

Employed as coloring agents in association with these polystyrene resins and/or ethyl cellulose are various pigments and dyes such as carbon blacks and azo oil blacks, e.g., Neo Spectra carbon black, Nigrosin black, Nigrosin spectra, Nigrosin SSB, Sudan Black CRA and the like.

The various types of colorants are well known in the art and need not be more specifically described here.

In accordance with the present invention, it has been found that the properties of these toner mixtures are greatly improved by the incorporation therein of about 1 to about 60 weight percent of sulfur 'which has the effect of. improving the properties of the toner in the electrostatic process, and also operates as an inexpensive filler for the mixture. It has been found that the use of sulfur as described herein gives rise to toner mixes having greatly improved triboelectric properties, greater thermostability, relatively sharp melting points at temperatures which are compatible with the backing material as well as improved adhesion of the toner to the backing material.

As pointed out the amount of sulfur to be incorporated into the toner mixture is about 1 to about 60 weight percent based on the total weight of the toner mixture. Commercially available sulfur may be employed as desired as no particularly critical preparatory treatments need be carried out prior to its incorporation into the mixture.

In formulating the toner mix, the polystyrene resin and/or ethyl cellulose, desired coloring agents and sulfur are preferably mixed and fused or melted together with agitation while heating at about 180 to 200 C. for several hours, preferably about 1 to 5 hours. During the reaction, it will be observed that hydrogen sulfide gas is evolved thus indicating that a reaction with sulfur is occurring. However, the nature of this reaction is not known. After the fusion is completed, the hot product may be poured onto a stainless steel tray, cooled and ground into fine particles. Thereafter the ground mixture may be conditioned by ball milling for about 48 to 96 hours.

A preferred formulation for production of the toner mixture of this invention would include the following amounts of the several materials by weight:

In forming the developer mix for use in the electrostatic process, about 1 to 3 percent, preferably 2 percent, of the resultant sulfur-containing toner mixture 1s 1ntimately admixed with about 97 percent to 99 percent of a carrier such as iron powder, which is commercially available as a carrier for use in dry electrostatic copying processes. This mixture has been found particularly suitable for use in electrostatic processes employing magnetic brush development copying.

The following working examples are presented to illustrate the invention with respect to certain preferred embodiments thereof but are not to be considered as limitative thereon.

EXAMPLE I A toner formulation according to this invention was prepared as follows:

Ingredient: Amount, grams Ethyl cellulose 20 Low molecular weight polystyrene (Piccolastic D75) 75 Neo spectra carbon black 1 Sudan black CRA 4 Sulfur 2 EXAMPLE II A second toner formulation was prepared as follows:

Ingredient: Amount, grams Ethyl cellulose 20 Low molecular weight polystyrene (Piccolastic D75) 75 Neo spectra carbon black 1 Sudan black CRA 5 Sulfur 20 The components of the mixture were mixed, melted and conditioned as in Example I to yield the toner particles.

EXAMPLE III A third toner formulation was prepared as follows:

Ingredient: Amount, grams Ethyl cellulose 20.0 Low molecular weight polystyrene (Piccolastic D75) 75.0 Neo spectra carbon black 1.0 Sudan black 7.5

Sulfur 40.0

The components were mixed, melted and treated as in Example I to yield the toner particles.

In production of the above toner, Piccolastic D75 is a low molecular weight polystyrene having a melting point of 75 C. and a molecular weight of about 800 and is available from the Pennsylvania Industrial Chemical Corporation of Clairton, Pa.

EXAMPLE IV Using the above-prepared toner formulations, the following developer mixtures were prepared by intimately admixing each toner with particles of an iron carrier.

Developer A.20 grams of toner from Example I; 800 grams of iron particles.

Developer B.20 grams of toner from Example 11; 800 grams of iron particles.

Developer C.20 grams of toner from Example III, 800 grams of iron particles.

Mixtures A, B, and C were separately mixed at room temperature and evaluated in an electrostatic copying machine. It was observed that the developer mixes had good thermostability and printing performance exceeding toners which do not contain sulfur.

The iron particles used in formulating Developers A, B and C was an iron powder identified as No. 625 obtained from Hoeganaes Corporation of Riverton, N]. All of this material passed through a mesh screen, about 40% by weight passed through a 200 mesh screen and about 20% by weight passed through a 325 mesh screen. This latter (rfine) portion would have a particle size corresponding roughly to from 35 to 40 microns. It will be understood, however, that other grades of iron particles of relatively small size, i.e., being in their greatest dimension about 0.001 inch to 0.020 inch and preferably from about 0.002 inch to 0.008 inch may be used. Such iron powders should be free from grease and other materials which are soluble in alcohol.

While the present invention has been described with particular reference to certain preferred embodiments thereof, it is to be understood that the invention is to be considered as limited only by the appended claims.

What is claimed is:

1. A developer mix useful for developing latent electrostatic images, said mix consisting essentially of about 1 to about 3 weight percent of a toner and the remainder consisting essentially of a carrier comprising iron particles, said toner consisting essentially of loose, movable particles of an electrostatic attractable powder reaction product formed by fusing of (l) a member selected from the group consisting of (a) ethyl cellulose, (b) polystyrene having a molecular weight of about 350 to 5,000 and a melting point of about 50 to 250 C. and (0) mixtures thereof; (2) a coloring agent selected from the group consisting of carbon blacks and azo oil blacks, and (3) about 1 to 60 weight percent of sulfur based on the total weight of the toner, at a temperature of about 180 to 200 C., cooling and grinding into said loose movable particles.

2. A developer mix according to claim 1 wherein the toner mixture is present in an amount of about 1.5 to 3 parts based on the total weight of the developer composition.

3. A developer mix according to claim 2 wherein the toner contains about 2 to 10 weight percent of the coloring agent.

4. A developer mix according to claim 3 wherein, said toner contains a mixture of polystyrene and ethyl cellulose.

5. A developer mix according to claim 4 wherein the toner mixture consists essentially of about 30 to 80 weight percent of polystyrene, 5 to 30 Weight percent of ethyl cellulose, 2 to 10 weight percent of coloring agent, and 1 to 60 weight percent of sulfur.

References Cited UNITED STATES PATENTS 10/ 1966 Blanchette et a1 252-62.1 1/1966 Sciarnbi 25262.1

11/1965 Johnson 25262.1 1/ 1965 Tomanek et a1 252-621 10/1963 Evans 252--62.1

US. Cl. X.R. 

